While chicken is cooking in oven, place chicken stock and bouillon cube in deep saucepan; bring to a simmer. Add spinach leaves and watercress, stirring and tossing well until wilted, about 2 -3 minutes. Remove greens from pan, reserving stock. Drain greens on paper towels. Boil stock and cook until liquid is reduced and begins to thicken, about 5 minutes. Whisk in one tablespoon butter.